MicrophoneArray.append(MicrophoneArray), do not crash when signal exists

[bogdad]

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hey, thank you for pyroomacoutics!

this pull request fixes a small issue i bumped into when trying to use it in my toy example, when i have few microphones and they are added sequentionally at different points in time with the following stacktrace:

2025-07-21 11:47:12,185 - simulator.virtual_speaker - ERROR - Error processing audio chunk: 'MicrophoneArray' object has no attribute 'shape'
2025-07-21 11:47:12,185 - simulator.virtual_speaker - ERROR - Full traceback: Traceback (most recent call last):
  File "/.../simulator/virtual_speaker.py", line 117, in _process_audio_chunk
    await self.speaker_server.room_sim.update_simulation()
  File "/.../simulator/room_simulation.py", line 145, in update_simulation
    self.room.add_microphone_array(mic_positions.T)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/.../simulator/venv/lib/python3.13/site-packages/pyroomacoustics/room.py", line 2115, in add_microphone_array
    return self.add(mic_array)
           ~~~~~~~~^^^^^^^^^^^
  File "/.../simulator/venv/lib/python3.13/site-packages/pyroomacoustics/room.py", line 2018, in add
    self.mic_array.append(obj)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^
  File "/.../simulator/venv/lib/python3.13/site-packages/pyroomacoustics/beamforming.py", line 551, in append
    (locs.shape[1], self.signals.shape[1]), dtype=self.signals.dtype
     ^^^^^^^^^^
AttributeError: 'MicrophoneArray' object has no attribute 'shape'

I have not nailed the example yet, maybe the my usage is borked and i should not want to do this, but this still seems like a correct fix.

pyright seems to find this too.

added a test which does fail before and passes after.

thank you again!